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Creating Patterned Multispectral Filters

Tuesday, 01 July 2008

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In recent years the explosion in demand for multispectral imaging has coupled with the industry’s insatiable need for weight reduction, thereby greatly increasing the demand for more sophisticated approaches to producing optical filters that are used in these systems. One method to meet the challenge of reducing the weight of a multispectral system is to eliminate beam-splitting optics and multiple detectors by patterning a filter array on a single substrate, or directly on the CCD itself.

Traditional Methods

Metal contact masking is the traditional method for applying one or more coatings in patterned form on a single substrate. For many applications, metal masks made from materials such as stainless steel are relatively inexpensive to fabricate, easy to use, and are capable of withstanding the in-vacuum process conditions associated with the vacuum deposition of stable and durable optical coatings. However, for more sophisticated applications, such as multispectral patterning on a single substrate or directly on the CCD, metal masks are expensive to manufacture and downgrade the substrate if allowed to come in contact with it during coating. Metal masks are also difficult to align with the substrate. Alignment is especially challenging when producing a deposited pattern that can be cleanly aligned so its edges interface with the edges of existing patterns without gaps or overlaps.

Other methods for producing multispectral filters are similarly limiting. Coating individual substrates, dicing these substrates to required dimensions, and then bonding them to a substrate to form a multispectral array is time-consuming, costly, and limited by the size constraints of the processes involved. Similarly, multispectral filters produced using colored glass or gels are not very durable and they limit the designer to the catalog of available color glasses and gels. The semiconductor industry has achieved finely detailed patterns using direct etch photolithography and ion etching. While these techniques work exceedingly well for silicon or siliconbased materials, they are not effective in patterning thick optical coatings consisting of multilayer stacks of two or more all-dielectric coating materials.

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